Battery chargers are used in a variety of applications for charging wet or dry batteries. When charging wet batteries there is a risk for explosions when connecting the battery charger to the battery since hydrogen gas generated by the electrolyte can explode if ignited by sparks from the connection. This may of course be solved by having the battery charger switched-off during connection, but a user may forget this, and for some applications there is a desire to have an automatic start of the charging process when the battery is connected.
The patent document U.S. Pat. No. 5,045,769 describes a battery charging system where sparking is prevented in that there is a time delay of applying charging current to the battery to allow the battery terminals to be secured. The voltage over the battery terminals is measured and if there is stated that there is a voltage present a delay circuit is activated, which after a certain time, e.g. 2 seconds, closes a switch connecting the charger to the battery. However, should the power supply fail for some reason, there is a risk that the battery is drained because the energy for controlling the spark protection circuit, and also other functions of the charger according to the document is taken from the battery.